Substitutes for fossil fuels are of importance due to the diminishing fossil fuel reserves and increasing atmospheric CO2 levels. Biomass is a promising substitute in many applications where fossil fuels have traditionally been used. Biomass may be converted into transportation fuels, as well as used in the chemical industry for the production of fine chemicals, such as acrylic plastics. Furthermore, biomass is both a renewable and CO2-neutral source.
However, in contrast to conventional fossil fuels such as petroleum based feedstock, biomass is a lignocellulosic feedstock which has a high oxygen to carbon mole ratio due to oxygenated groups such as —OH, —OR (where R denotes a carbon chain), —C═O, and —COOH. The oxygen/carbon ratio results in uncontrolled decomposition with temperature, as well as low volatility, high reactivity, and high solubility in water.
Thus, in contrast to the conventional fossil feedstock, biomass cannot be used directly in the main streams for fuel or for the chemical industry. Biomass derived molecules therefore needs to be further processed to reduce the oxygen content. A known route to de-functionalise molecules is hydrogenolysis.
The biomass derived molecules include alpha-hydroxy acids and esters (also written as α-hydroxy) such as lactic acid and alkyl esters of lactic acid, e.g. alkyl lactates, such as methyl lactate. In an alpha-hydroxy acid or esters, the hydroxyl group is attached to the carbon atom carrying the carboxyl or carbonyl group. The hydrogenolysis of the alpha-hydroxy esters as lactic acid or alkyl lactates may result in alkyl propionate, such as methyl propionate. Methyl propionate is also known as an important chemical precursor in the production of acrylic plastics. The hydrogenolysis of methyl lactate to methyl propionate is illustrated in reaction (R-I).

Methyl propionate is traditionally not produced based on biomass and hydrogenolysis. Instead it has been based on fossil feedstock molecules, such as the methoxycarbonylation of ethylene with carbon monoxide (CO), methanol, and a homogeneous palladium (Pd)—phosphine complex as catalyst [1]. However, the traditional production process suffers from drawbacks such as the requirement for poisonous gas (CO), the expensive catalyst materials, and in many cases, the dependence on fossil feedstock molecules.
Thus, the synthesis of methyl propionate by alternative methods such as hydrogenolysis of biomass derived molecules is receiving increasing interest. Xiu et al. [2] described the hydrogenolysis of ethyl lactate, resulting in several different products including ethyl propionate. The hydrogenolysis is catalysed by a heterogeneous; cobalt (Co) based bimetallic catalyst made of Co-M, where M can be Zn, Fe, Cu, or Sn, and where the catalyst is supported by SiO2. The addition of Fe was disclosed to increase the selectivity to ethyl propionate, however the catalysts did not provide a high conversion or a high yield of the propionate.
Furthermore, heterogeneous catalysts of noble metals (such as Ru, Re), and other metals such as Ni, Cu, Fe, and Co have further been shown to catalyse hydrogenolysis processes, e.g. the hydrogenolysis of polyols. However, an efficient catalytic process for the hydrogenolysis of alpha-hydroxy esters (such as alkyl lactate) to alkyl propionate has not been disclosed.
The production of methyl propionate by other alternative methods, such as enzymatic catalysis has also been described [3]. However, the methods were not efficient and showed low selectivity of methyl propionate.
References
[1] G. R. Eastham, B. T. Heaton, J. A. Iggo, R. P. Tooze, R. Whyman, S. Zacchini, Chem. Commun, 2000, 609
[2] J. Xiu et al., Chin. J. Catal., 2012, 33, 1642.
[3] H. L. van Beek, R. T. Winter, G. R. Eastham, M. W. Fraaije, Chem. Commun., 2014, 50, 13034.